Indicator pH measurement

Precipitation over North America gradually becomes more acidic from west to east, especially in industrialized areas of the Northeast. This acid rain may be a result of the release of nitrogen and sulfur oxides into the atmosphere. The colors and numbers (see key) indicate pH measured at field laboratories in 2004. Data from National Atmospheric Deposition Program/National Trends Network http //nadp.sws.uiuc.edu. 

HCI aql] mol/L Predicted pH pH measured with universal indicator pH measured with pH meter... 

Fig. 4. Effect of various pH and buffers on double-stranded DNA thermodegradation analyzed using neutral and alkaline gels. Plasmid pTZ18 was incubated for 20 min at 100° in various buffers at the indicated pH (measured at the incubation temperature with a pH prohe). After incubation, samples were divided into two parts and run in either neutral or alkaline gels. RC, Random-coiled molecules.

The equilibrium constant for an acid-base indicator is determined by preparing three solutions, each of which has a total indicator concentration of 1.35 X lQ-5 M. The pH of the first solution is adjusted until it is acidic enough to ensure that only the acid form of the indicator is present, yielding an absorbance of 0.673. The absorbance of the second solution, whose pH was adjusted to give only the base form of the indicator, was measured at 0.118. The pH of the third solution was adjusted to 4.17 and had an absorbance of 0.439. What is the acidity constant for the acid-base indicator ... 

Because they are weak acids or bases, the iadicators may affect the pH of the sample, especially ia the case of a poorly buffered solution. Variations in the ionic strength or solvent composition, or both, also can produce large uncertainties in pH measurements, presumably caused by changes in the equihbria of the indicator species. Specific chemical reactions also may occur between solutes in the sample and the indicator species to produce appreciable pH errors. Examples of such interferences include binding of the indicator forms by proteins and colloidal substances and direct reaction with sample components, eg, oxidising agents and heavy-metal ions. 

Measurement of some of these parameters identifies the risk of a particular type of corrosion, for example pH measurements assess the risk of acid attack and redox potential measurements is used to assess the suitability of the soil for microbiological corrosion, a low redox potential indicates that the soil is anaerobic and favourable for the life cycle of anaerobic bacteria such as to sulphate-reducing bacteria. Other measurements are more general, resistivity measurements being the most widely quoted. However, as yet no single parameter has been identified which can confidently be expected to assess the corrosion risk of a given soil. It is therefore common practice to measure several parameters and make an assessment from the results. 

Among potentiometric methods of analysis that are important for ecological applications, the one most widely used is that of pH measurements with an indicator electrode whose potential is a function of the hydrogen ion concentration. More recently, ion-selective electrodes reversible to other cations such as those of heavy metals have become available. 

As in normal potentiometry one uses and indicator electrode versus a reference electrode, the electrodes should, especially in pH measurements, be those recommended by the supplier of the pH meter in order to obtain a direct reading of the pH value displayed. In redox or other potential measurements any suitable reference electrode of known potential can be applied. However, a reference electrode is only suitable if a junction potential is excluded, e.g., an Ag-AgCl electrode in a solution of fixed Ag+ concentration or a calomel electrode in a saturated KC1 solution as a junction in many instances a direct contact of Cl" with the solution under test (possibly causing precipitation therein) is not allowed, so that an extra or so-called double junction with KN03 solution is required. Sometimes micro-electrodes or other adaptations of the surface are required. 

When the log /J/pH measurement of a peptide is performed by the shake-flask or the partition chromatography method (using hydrophilic buffers to control pH), usually the shape of the curve is that of a parabola (see Ref. 371 and Fig. 1 in Ref. 282), where the maximum log I) value corresponds to the pH at the isoelectric point (near pH 5-6). Surprisingly, when the potentiometric method is used to characterize the same peptide [275], the curve produced is a step function, as indicated by the thick line in Fig. 4.5 for dipeptide Trp-Phe. 

Most probably, the first - but non-fiberoptic - sensors for continuous use where those for pH and for oxygen. It has been known for decades that cellulosic paper can be soaked with pH indicator dyes to give pH indicator strips which, however, leached and thus were of the "single-use" type. The respective research and development is not easily traced back since it is not well documented in the public literature. However, in the 1970s, indicator strips became available where they pH indicator dye was covalently linked to the cellulose matrix, usually via vinylsulfonyl groups. These "nonbleeding" test strips allowed a distinctly improved and continuous pH measurement, initially by visual inspection. In the late 1980 s instruments were made available that enabled the color (more precisely the reflectance) of such sensor strips to be quantified and related to pH. Respective instruments are based on the use of LEDs and are small enough to be useful for field tests in that they can be even hand-held. This simple and low cost detection system is still superior to many of the complicated, if not expensive optical pH sensors that have been described in the past 20 years. 

Table 1. Selected absorption indicators for measurements of acidic or basic pHs, and respective pKa values at room temperatures1.

Figure 10 shows the absorbance response of an indicator to pH change. As can be seen, with the use of an indicator, the pH measurements can be made only over a range of about two pH units, i.e. pKa 1. Beyond this range, the change in absorbance with pH becomes small and the error in pH measurements will be large. 

American scientists prepared the organo-silica sol-gel membranes60 and demonstrated in a single layer format for pH measurement and multiple-layer format for both C02 and NH3. The sensors used a hydroxypyrenetrisulfonic acid (HPTS) as the indicator immobilizes in a base-catalyzed sol-gel containing poly(dimethyl)siloxane, aminopropyl-triethoxysilane (APTES) and tetraethylorthosilicate (TEOS). This indicator gel was over coated with a hydrophobic sol-gel to reduce cross reactivity to pH when either carbon dioxide or ammonia were examined. 

The use of only one indicator to cover the whole range would simplify the whole system and recent studies indicate methyl red as feasible indicator for gastric pH measurement. The dye, immoliside on CPG was shown to be capable of covering the whole pH range between 1 and 827 28. 

Because standardization is an essential component of soil pH measurement, care must be taken to have good buffers. If the buffers show any indication of contamination, such as material floating in them, soil, or microbial growth, they... 

Lubbers DW, Opitz N, Speiser PP, Bisson HJ (1977) Nanoencapsulated fluorescence indicator molecules measuring pH and p02 down to submicroscopical regions on the basis of the optode principle. Z Naturforsch 32 133-134... 

Wolfbeis O. S., Furlinger E., Kroneis H. and Marsoner H. (1983) Fluorimetric Analysis. 1. A Study on Fluorescent Indicators for Measuring Near Neutral ( Physiological ) pH Values, Fresenius Z. Anal. Chem. 314, 119-24. 

Observations that the presence of protein affects the colour change of some indicators used in acid-base titrations led to the development of methods for the quantitation of proteins based on these altered absorption characteristics of such dyes. As the presence of protein alters the colour produced by these indicators when measuring pH, so in the quantitation of proteins using dye-binding methods the control of pH is vital. 

It is important to emphasize that these considerations also apply to indicators with other ionic specificities and provided that their spectra are well resolved from those of the pH indicators, simultaneous measurements of ion concentrations and pH are now commonly practiced.(118)... 

Our approximation in the calculation is 150 cal/mole, which is quite satisfactory. The fact that dolomite is the mineral phase nearest to complete equilibrium is in agreement with the geology of the area and was in fact already evident at first sight from the chemical analyses, indicating similar molalities for Ca and Mg in solution. The resulting /co2(j) 10 - , which is much higher than the atmospheric value. The previous recommendation about pH measurements is particularly obvious in this case. 

In this section we first describe the results of measurements made on fresh and aged solutions. Potentlometric, pH, measurements, which Indicate the extent of hydrolysis of the aluminum Ions, were made on all solutions during the aging process. Next, we report surface areas of pillared clays made from the aged solutions. Finally, we will discuss the cracking activities of some selected pillared clay samples. 

Figure 7. Residual activities of papain-digested CBH I (open symbols) and CBH II (closed symbols). The enzymes ( 180 /jM) were incubated (pH 5.0, 25°C) with 0.6 fiM papain (300 1). Aliquots (50 /d) were removed at times indicated to measure the Avicelase activities (A, A) or to measure their activities against a small, chromophoric substrate as described in the text (CNPL in the case of CBH I, -o- MeUmbGs in the case of CBH II, - -). Residual activities are given as percent of the original.

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